1. Field of the Invention
This invention relates in general to accessing databases on a network, and more specifically, to translating eXtensible Markup Language (XML) queries into queries against a relational database for the purpose of publishing relational data as XML.
2. Description of Related Art
Computer systems incorporating Relational Database Management System (RDBMS) software using the Structured Query Language (SQL) interface are well known in the art. The SQL interface has evolved into a standard language for RDBMS software and has been adopted as such by both the American Nationals Standard Organization (ANSI) and the International Standards Organization (ISO).
In an RDBMS, all data is structured as tables. The SQL interface allows users to formulate relational operations on the tables either interactively, in batch files, or embedded in host languages such as C, COBOL, etc. Operators are provided in SQL that allow the user to manipulate the data, wherein each operator operates on either one or two tables and produces a new table as a result. The power of SQL lies in its ability to join and relate information from multiple tables or views, and the ability to perform complex operations with a single statement.
An alternative data format to the tables found in an RDBMS is XML, which is a tag language for describing documents. XML is a restricted form of SGML, the Standard Generalized Markup Language defined in ISO 8879. XML is more fully described at the URL:                <www.w3.org/TR/1998/RCE-xml-19980210>        
An XML document has two parts: the marked up document, and the document schema. XML documents are made up of storage units called elements, which can be nested to form a hierarchical structure. Elements can contain either parsed or unparsed data. Parsed data is made up of characters, some of which form character data, and some of which form markup. Markup encodes a description of the document's storage layout and logical structure.
XML schemas specify constraints on the structures and types of elements in an XML document. The basic schema for XML is the DTD (Document Type Definition). Other XML schema definitions are also being developed, such as XML Schema, which is described at the URL:                <www.w3.org/TR/1999/WD-xmlschema-1-19990924>        
Several query languages have been proposed for searching and reformatting XML documents. One such language is XML-QL, which is described at the URL:                <www.w3.org/TR/NOTE-xml-ql>        
In XML-QL, queries have the general form:                WHERE <pattern>        IN <URLs of input documents>        CONSTRUCT <output XML>        
The input documents are searched for the pattern in the WHERE part of the query. Data bindings, which are specified in the pattern, are used to construct the output XML. Note that the output XML does not have to be a well-formed document with matching start and end tags, nor does it have to conform to any DTD. Important operational features of XML-QL include:                Support for navigation in the search pattern of a query. This navigation can be expressed using path expressions.        The ability to join and relate information from multiple input documents in a single query.        Support for nested subqueries in the CONSTRUCT part of a query.        The ability to generate output XML with a different structure than the input documents.        
Other XML query languages that have been proposed generally support only a subset of the above operational features. For example, the XML Query Language (XQL) does not support joins, subqueries, or the ability to generate output with a different structure than its input document. XQL is described at the URL:                <www.w3.org/tandS//QL/QL98/xql.html>        
XML is likely to become a future standard for information exchange between peer data stores, and between client visualization tools and data servers. Most business data will probably continue to be stored in relational database systems, however. Thus, there is a need for a mechanism to publish and query relational data as XML.